That’s up to you!
If you want a Gravatar with each comment, you’ll have to find the page in your template that generates the comments.
In the default theme it is comments.php.
You will find the loop that loops through and presents all comments for a post. It starts like this:
<?php foreach ($comments as $comment) : ?>
Identify where you want the Gravatar to appear inside the loop.
( Someone else may give better instructions )